1. Field of the Invention
The present invention relates to an optical element such as a retardation plate, a polarizing plate, or a color filter for display, in particular, an optical element comprising, as a refractive index anisotropic layer, a liquid crystal layer comprising a liquid crystal composition containing a polymerizable liquid crystal material; and a method for manufacturing the same. In the specification and the claims, the term “liquid crystal layer” means a layer having an optical liquid crystal property, and the state of the layer may be a solid phase state that the molecular arrangement which a liquid crystal phase has is kept and solidified.
2. Description of the Related Art
In general, liquid crystal is used as a display medium, typical examples of which include TN (twisted nematic) and STN (super twisted nematic) type display elements, by use of reversible mobility of the molecular arrangement thereof. Besides, liquid crystal is used as an optical element, such as a retardation plate, a polarizing plate or a color filter for display, by use of the alignment and the refractive index anisotropy thereof.
In connection with the latter usage, in recent years, there have been proposed many optical elements which each has, as a refractive index anisotropic layer, a liquid crystal layer made of a polymerizable liquid crystal material. Specifically, for example, Japanese Patent Application National Publication No. 2002-533742 describes an optical element having functions of wavelength-selective reflectivity and polarization-selective reflectivity and is produced by use of a special polymerizable liquid crystal compound. Japanese Patent Application Laid-Open (JP-A) No. 5-215921 describes a birefringent plate produced by use of a polymerizable liquid crystal compound having a bar-like structure. Furthermore, JP-A Nos. 8-338913 and 9-152509 each describes an optical compensation sheet produced by use of a polymerizable liquid crystal compound having a disc-like structure.
Incidentally, as such an optical element, there is generally used an optical element in which a liquid crystal layer (a refractive index anisotropic layer) made of a polymerizable liquid crystal material is laminated over a support, such as a plastic film, so as to interpose an alignment layer therebetween.
The alignment layer arranged between the support and the refractive index anisotropic layer has alignment limiting force for limiting the alignment direction of liquid crystal molecules in the refractive index anisotropic layer. Such an alignment layer can be formed by forming a layer of a polymer (such as polyimide, polyvinyl alcohol, or gelatin) having alignment property onto a support, and then subjecting the polymer layer to alignment process such as rubbing treatment. When rubbing treatment is applied, as alignment process, to an alignment layer, static electricity or dust is generated in or on the surface of this layer; accordingly, methods have been researched for causing an alignment layer to express alignment limiting force without conducting rubbing treatment. One of them is a photo alignment method in which light in an arbitrary polarized light state (polarized light) is radiated onto an alignment layer, thereby generating alignment limiting force (anisotropy) on the surface of the alignment layer. The photo alignment method is classified into “photo isomerization”, in which only the shape of molecules is changed to vary the alignment state thereof reversibly, and “photo reaction”, in which molecules themselves are changed. The “photo reaction” of the latter is further classified into dimerization , decomposition, coupling, decomposition-coupling, and others.
About the above-mentioned optical element, which comprises a support, an alignment layer and a refractive index anisotropic layer, it is undoubtedly important that the optical property of each layer of the optical element is good. Besides, it is also important that the adhesive property between the layers of the optical element or the endurance of each of the layers is good. In connection with the latter subject, for example, if the adhesive property between an alignment layer and a refractive index anisotropic layer which constitute an optical element is poor, there arises a problem that the refractive index anisotropic layer is easily peeled from the alignment layer. When the optical element is used or stored at high temperature and high humidity, there also arises a problem that netlike shriverings are generated in the refractive index anisotropic layer.
In order to solve such problems, methods described below have been proposed hitherto.
JP-A No. 9-152509 proposes a method of using a modified polyvinyl alcohol as the material of an alignment layer and improving the adhesive property between the alignment layer and a refractive index anisotropic layer by chemical bond in the interface between the two layers. JP-A No. 10-10320 proposes a method of inserting an anchor coat layer between layers having a low adhesive property with each other, thereby improving the adhesive property between these layers. Furthermore, Japanese Patent Application National Publication No. 2000-514202 proposes a method of adding a monomer into a material for a refractive index anisotropic layer, thereby improving the endurance of the refractive index anisotropic layer. Specifically, there is proposed a method of incorporating, into a reactive mesogen compound as a material for a refractive index anisotropic layer, a non-mesogen compound having 2 or more polymerizable functional groups in an amount of 20% or less of the mesogen compound, thereby changing the glass transition temperature, the thermal stability and the mechanical stability of the layer.
Of the above-mentioned methods in the prior art, however, the method described in JP-A No. 9-152509 has the following problem: the boiling point of a solvent used for the modifying reaction of the polyvinyl alcohol is high and a solution containing this solvent cannot be used, as it is, as a coating solution; accordingly, a step of purifying the polyvinyl alcohol by the re-precipitation thereof is indispensable, so that costs for the production increase. The method described in JP-A No. 10-10320 has a problem that when a liquid crystal compound is used as the material of the refractive index anisotropic layer, the liquid crystal molecules are not satisfactorily aligned on the anchor coat layer. The method described in the Japanese Patent Application National Publication No. 2000-514202 has the following problem: when the alignment state of the liquid crystal molecules is fixed after the alignment process thereof so as to make the liquid crystal into a refractive index anisotropic layer, additives become impurities when the liquid crystal molecules are aligned; thus, the alignment of the liquid crystal molecules is hindered, so that the optical property thereof lowers (for example, display unevenness is generated.)